1. Field of the Invention
The present invention relates generally to a stationary blade of a steam turbine and more particularly to a stationary blade applied as a precision cast hollow blade which is used for a final stage of a steam turbine and has an enhanced drain removal efficiency of its blade surface.
2. Description of the Prior Art
As for a final stage stationary blade of a steam turbine, steam which has worked on a front stage side thereof passes through it and as the temperature of the steam becomes lower, the blade is not specifically cooled. Although drain comes out on a blade surface, it is a present situation that there are provided slits for recovering the drain through which the drain is led into a blade interior to be recovered through a hollow therein.
FIG. 5 is a perspective view of a precision cast hollow blade used in a steam turbine in the prior art, as mentioned above. In FIG. 5, numeral 2 designates an outer shroud, numeral 3 designates an inner shroud and numeral 31 designates a stationary blade, which is a precision cast blade to be used for a final stage of the steam turbine and has a hollow formed therein. Numeral 32 designates the hollow in the blade interior, which is formed as a single hollow. Numerals 33, 34, respectively, designate a plurality of ventral slits, which are provided in the blade surface linearly with a predetermined interval between each of the slits so that two rows of the ventral slits are arranged in parallel with each other. Numeral 35 designates a plurality of dorsal slits provided likewise in the blade surface. The ventral slits 33, 34 and the dorsal slits 35 are provided to open at the blade surface and communicate with the hollow 32 so that the drain may flow therethrough into the blade interior.
FIG. 6 is a cross sectional view taken on line G--G of FIG. 5. As mentioned above, the single hollow 32 is formed in the stationary blade 31 and the ventral slits 33, 34 are provided in a ventral portion of the stationary blade 31 to pass through from the blade surface to the hollow 32. Likewise, the dorsal slit 35 is provided in a dorsal portion of the stationary blade 31 to pass through from the blade surface to the hollow 32.
In the steam turbine stationary blade constructed as mentioned above, drain 50 on the blade surface flows into the ventral slits 33, 34 and the dorsal slits 35 to be led into the hollow 32 and then passes through the outer shroud 2 or the inner shroud 3 to be recovered into a condenser (not shown) via a pipe system (not shown). In the process of such flow of the drain 50 which has been led into the hollow 32 through the ventral slits 33, 34 and the dorsal slits 35, there is a problem in operation that the drain so led into the hollow 32 through the dorsal slits 35 moves along an inner wall of the hollow 32 to flow out again through the ventral slits 33, 34 or a reverse case thereof occurs, and an improvement in terms of a drain removal efficiency is desired.
That is, in the precision cast hollow stationary blade of a steam turbine in the prior art as mentioned above, there occurs a case that the drain recovered into the hollow 32 through the dorsal slits 35, or through the ventral slits 3334, moves along the inner wall of the hollow 32 to flow out again through the ventral slits 33, 34, or through the dorsal slits 35, respectively, which results in lowering the drain recovery efficiency.